Process of forming transparent conductive coatings with sintering additives

ABSTRACT

A process is disclosed for the delayed sintering of metal nanoparticles in a self-assembled transparent conductive coating by incorporating a sintering additive into the water phase of the emulsion used to form the coating. The sintering additive reduces the standard reduction potential of the metal ion of the metal forming the nanoparticles by an amount greater than 0.1V but less than the full reduction potential of the metal ion. Emulsion compositions used in the process are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 13/583,563filed Oct. 22, 2012, which is a National Stage application under 35U.S.C. § 371 of International Application No. PCT/IB2011/000765, havingan International Filing Date of Mar. 9, 2011, which claims priority toU.S. Provisional Application Ser. No. 61/311,992, filed on Mar. 9, 2010.The disclosures of the prior applications are considered part of (andare incorporated by reference in) the disclosure of this application.

TECHNICAL FIELD

This invention relates to a process of forming transparent conductivecoatings comprising a pattern of conductive traces formed of at leastpartially joined nanoparticles defining randomly-shaped cellstransparent to light. More particularly, it relates to a process offorming transparent conductive coatings by the self-assembly ofnanoparticles from a coated emulsion into conductive traces that definerandomly-shaped cells that are transparent to light. Coatingcompositions for forming the transparent conductive coatings are alsodescribed.

BACKGROUND

Transparent conductive coatings are useful in a variety of electronicsdevices. These coatings provide a number of functions such aselectromagnetic (EMI shielding) and electrostatic dissipation, and theyserve as light transmitting conductive layers and electrodes in a widevariety of applications. Such applications include, but are not limitedto, touch screen displays, wireless electronic boards, photovoltaicdevices, conductive textiles and fibers, organic light emitting diodes(OLEDs), electroluminescent devices, and electrophoretic displays, suchas e-paper.

Transparent conductive coatings such as those described in U.S. Pat.Nos. 7,566,360 and 7,601,406 and WO2006/135735 are formed from theself-assembly of conductive nanoparticles coated from an emulsion onto asubstrate and dried. Following the coating step, the nanoparticlesself-assemble into a network-like conductive pattern of randomly-shapedcells that are transparent to light.

In order to achieve low sheet resistances on the order of 100 ohm/sq orless, the coatings typically require sintering after pattern formation.Such sintering can be done by thermal treatment alone, but thetemperatures required are generally too high for most flexible,polymeric substrates that are desirably used in commercial scaleroll-to-roll processing. Sintering can also be done chemically by aseparate processing step such as exposing the formed pattern to certainchemical washes or vapors. Examples include exposure to an acid orformaldehyde solution or vapor, as disclosed in U.S. Pat. Nos. 7,566,360and 7,601,406, or to acetone or other organic solvents as disclosed inPCT/US2009/046243. Such separate chemical processing steps are costly,inconvenient and potentially hazardous to workers in a commercial scaleproduction process.

SUMMARY OF THE INVENTION

The process and coating compositions of the present invention eliminatethe need for a separate chemical sintering step in the formation of lowresistance transparent conductive coatings from nanoparticle-containingemulsions.

A process is disclosed for forming a transparent conductive coating on asubstrate comprising: (1) forming an emulsion by mixing together (a) anoil phase comprising a solvent that is non-miscible with water havingdispersed therein metal nanoparticles and (b) a water phase comprisingwater or a water-miscible solvent and an additive for delayed sinteringof the nanoparticles that reduces the standard reduction potential ofthe metal ion of the metal forming the nanoparticles by an amountgreater than 0.1V but less than the full reduction potential of themetal ion; (2) applying the emulsion to a substrate to form a wetcoating; and (4) evaporating the liquid from the coating to form a drycoating comprising a network of electrically-conductive traces thatdefine randomly-shaped cells that are transparent to light.

The sheet resistance of the coating is preferably less than 100 ohm/sq,and most preferably less than 10 ohm/sq without further chemicalsintering.

In a preferred embodiment of the process, the additive for delayedsintering is an acid such as hydrochloric acid, sulfuric acid,phosphoric acid, acetic acid, or formic acid; a halide such as sodiumchloride, ammonium chloride, or potassium chloride; a halogenatedcompound such as quaternary ammonium salts or ionic liquids.

In a preferred embodiment, the pH of the water phase before the additionof any other ingredients in addition to the water or the water misciblesolvent and the delayed sintering additive is less than 3.0, and the pHof the water phase when mixed with the oil phase is greater than 8.0.

The invention also provides liquid coating compositions in the form ofan emulsion for forming a transparent conductive coating on a substratecomprising (a) an oil phase comprising a solvent that is non-misciblewith water having dispersed therein metal nanoparticles, and (b) a waterphase comprising water or a water-miscible solvent and an additive fordelayed sintering of the nanoparticles that reduces the standardreduction potential of the metal ion of the metal forming thenanoparticles by an amount greater than 0.1V but less than the fullreduction potential of the metal ion. In the preferred coatingcomposition, the additive for delayed sintering comprises an acid suchas hydrochloric acid, sulfuric acid, phosphoric acid, acetic acid, orformic acid; a halide such as sodium chloride, ammonium chloride, orpotassium chloride or a halogenated compound such as quaternary ammoniumsalts, or ionic liquids.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The process and coating materials of the present invention are used toform a transparent conductive coating on the surface of a substratecomprising a pattern of conductive traces formed of collections of atleast partially joined metal nanoparticles. Such traces define cells,generally free of the metal nanoparticles, which cells are generallytransparent to light. Articles comprising such a transparent conductivelayer are described in U.S. Pat. No. 7,601,406, the disclosure of whichis incorporated herein by reference.

The term “nanoparticles” as used herein refers to fine particles smallenough to be dispersed in a liquid to the extent they can be coated andform a uniform coating. This definition includes particles having anaverage particle size less than about three micrometers. For example, insome embodiments, the average particle size is less than one micrometer,and in some embodiments the particles measure less than 0.1 micrometerin at least one dimension.

The phrase “transparent to light” generally indicates lighttransparencies of between 30% and 95% in the visible wavelength range ofabout 400 nm to 700 nm.

A liquid emulsion wherein the metal nanoparticles are contained in thecontinuous phase of the emulsion is used to form the transparentconductive layer. The continuous phase evaporates faster than thediscontinuous phase, allowing growth of discontinuous phase cells tooccur by emulsion droplet coalescence in a controlled fashion. Drying ofthe emulsion produces the pattern comprising distinct light-transmittingcells surrounded by traces that transmit significantly less light thanthe light-transmitting cells. The pattern produced by the cells and theperipheral traces has a network-like character that is observable by alight microscope.

The liquid emulsion in accordance with the present invention is awater-in-oil emulsion, where the continuous phase comprises an organicsolvent having the nanoparticles dispersed therein, and thediscontinuous phase comprises water or a water-miscible solvent and theadditive for delayed sintering. Suitable solvents for use in formulatingthe emulsions are disclosed in U.S. Pat. No. 7,566,360, the disclosureof which is incorporated by herein by reference. Examples of preferredorganic solvents for the oil phase include those that are characterizedby an evaporation rate higher than the evaporation rate of water atambient conditions. The solvents can be selected from the group of atleast, but not limited to, petroleum ether, hexanes, heptanes, toluene,xylene, benzene, dichloroethane, trichloroethylene, chloroform,dichloromethane, nitromethane, dibromomethane, cyclopentanone,cyclohexanone, cyclohexane, cyclohexanol, UV and thermally curablemonomers (e.g., acrylates), or any mixture thereof. The water phase ispreferably based on water, but water-miscible solvents may also be usedalone or in combination with water. Examples of water-miscible solventsinclude, but are not limited to, methanol, ethanol, ethylene glycol,glycerol, dimethylformamide, dimethylacetamide, acetonitrile,dimethylsulfoxide, N-methylpyrrolidone or any mixture thereof.

Other additives may also be present in the oil phase and/or the waterphase of the emulsion formulations. For example, additives can include,but are not limited to, reactive or non-reactive diluents, oxygenscavengers, hard coat components, inhibitors, stabilizers, colorants,pigments, IR absorbers, surfactants, wetting agents, leveling agents,flow control agents, thixotropic or other rheology modifiers, slipagents, dispersion aids, defoamers, humectants, and corrosioninhibitors. Binder or adhesion components may also be present in theformulation, for example thermally-activated or UV-activated binders oradhesion promoters.

The metal nanoparticles may be comprised of conductive metals or mixtureof metals including metal alloys selected from, but not limited to, thegroup of silver, gold, platinum, palladium, nickel, cobalt, copper orany combination thereof. Preferred metal nanoparticles include silver,silver-copper alloys, silver palladium or other silver alloys or metalsor metals alloys produced by a process known as Metallurgic ChemicalProcess (MCP) described in U.S. Pat. Nos. 5,476,535 and 7,544,229. Inthe case of alloys, the “reduction potential” refers to the reductionpotential of the metal ion corresponding to the predominant metal, interms of weight percent, of the nanoparticles.

The metal nanoparticles mostly, though not necessarily exclusively,become part of the traces of the conductive network. In addition to theconductive particles mentioned above, the traces may also include otheradditional conductive materials such as metal oxides (for example ATO orITO) or conductive polymers, or combinations thereof. These additionalconductive materials may be supplied in various forms, for example, butnot limited to particles, solution or gelled particles.

The basic emulsion formulations also include emulsifying agents orbinders to stabilize the emulsion as described in U.S. Pat. No.7,566,360. Examples of emulsifying agents include non-ionic and ionicsurfactants such as the commercially available SPAN-20, span-80,glyceryl monooleate and dodecylsulfate. Examples of suitable bindersinclude modified cellulose such as ethyl cellulose (MW 100,000-200,000),modified urea such as the commercially available BYK-410, Byk-411, andBYK-420 from BYK Chemie Ltd.

The basic emulsion formulations as described in U.S. Pat. No. 7,566,360generally comprise between 40 and 80 percent of an organic solvent ormixture of organic solvents, from 0 to 3 percent of a binder, 0 to 4percent of an emulsifying agent, 2 to 10 percent of metal powder and 15to 55 percent of water or water miscible solvent.

The mixture may be prepared by dissolving the emulsifying agent and/orbinder in the organic solvent or mixture of organic solvents and addingthe metal powder. The metal powder is dispersed in the organic phase byultrasonic treatment, high shear mixing, high speed mixing or any othermethod commonly used for the preparation of a suspension. After thewater phase is added, a W/O emulsion is prepared by ultrasonictreatment, high shear mixing, high speed mixing or any other methodcommonly used for the preparation of an emulsion.

According to the present invention, the basic emulsion formulation isaltered by the addition of a delayed sintering additive to the waterphase of the emulsion. The term “delayed” as used herein means that asubstantial amount of the sintering attributed to the presence of theadditive occurs after the emulsion has been applied to the substrate.

The sintering-additive is a compound or mixture of compounds thatreduces the standard reduction potential of the metal ion of the metalforming the nanoparticles by an amount greater than 0.1 V but less thanthe full reduction potential of the metal ion. While not wishing to bebound by a particular theory, it is believed that delayed sintering isdue, at least in part, to the increased formation of metal ions (M+) atcertain areas of the conductive network followed by the diffusion ofthese M+ ions to areas of the conductive network having is a lowerconcentration of the M+ ions where the ions are reduced back to the bulkmetal state.

Nanoparticles of metals such as silver have different redox potentialsthan the bulk metal. This may allow sintering of a nanoparticle metalnetwork by use of chemical agents.

The redox potential of metal nanoparticles varies with the size of thenanoparticle, the radius of curvature of a local part of thenanoparticle, the specific crystal facet of the particle that isexposed, etc. The size of the specific energy barrier between ionic andneutral atomic states (quantitatively associated with the standardpotential) dictates the degree to which a metal in equilibrium isconverted into associated metal ions. Upon oxidation, a metal ion (M+)may diffuse in an aqueous environment. If a large number of M+ ions arebeing generated locally on a part of a series of electrically-connectedmetal nanoparticles, they will diffuse to an area in which a smallernumber are being generated. For instance, an area of very small radiusmetal nanoparticles will bias a natural local chemical reaction to makea relatively large concentration of M+ ions, whereas an area of largeradius metal nanoparticles may make a low concentration of ions. As ionsfrom the small radius area diffuse to an area of larger radius, thelocal potential at the larger radius area will disproportionately favorsubsequent reduction of the ions back into the bulk metallic state. Aslong as the metal system can allow electrical transport to neutralizethe incident M+ ion at the new surface, this net flux can continueindefinitely, with a slow overall consumption of metal from areas ofsmall radius of curvature (sharp areas) and a metal growth of areas oflow radius of curvature (flat areas). In general, this can lead tobetter electrical connection of weakly connected metal nanoparticles, asit fills in gaps between particles.

Changing the redox energies in the system can speed this process.Various chemistries can be chosen to increase the amount of ionicspecies in the system relative to the bulk metal. If the overall redoxlevels strongly favor metal being left in the non-ionized state, fewions are generated, and only slow sintering can occur. In contrast, ifthe overall redox levels completely bias formation of just ions, minimalre-precipitation of a bulk metal will occur, and the underlying metalwill be consumed (for instance, to the point of effective completeexhaustion of the metal).

To promote sintering at commercially useful speeds, a sintering additiveshould be selected that increases the formation of metal ions at smallradius areas and drives oxidation of the ions to the metal at largerradius areas. This will generally occur if the sintering additivereduces the standard reduction potential of the metal ion of the metalforming the nanoparticles by an amount greater than 0.1 V, but less thanthe full reduction potential of the metal ion. For example, in the caseof silver, the sintering additive should reduce the standard reductionpotential of the Ag+ ion by an amount greater than 0.1 V, but less thanthe full amount of a Ag+ ion which is 0.8V.

During the overall emulsion preparation-feeding-coating-patternformation-sintering process, the process needs to be tuned to preventunwanted reactions from occurring at inopportune times, e.g., too earlyin the process. For instance, it is undesirable for silver nanoparticlesto start evolving into connected agglomerates in a coating on theinterior of mixing equipment prior to coating on the intended surface.It is preferred for sintering to occur most aggressively after networkformation. Sintering before that point is likely to grow nanoparticlesto sizes that prevent patterns to form as intended.

Thus, additives should be selected that provide sintering after patternformation on the substrate and reduce the risk of unwanted earlysintering. If sintering actively occurs throughout the emulsion making,storage, delivery, and network formation stages, the metallicnanoparticles may grow to form agglomerates or macroparticles prior tonetwork formation, and thus obstruct network formation as larger sizedparticles will diffuse and transport differently in an emulsionenvironment. Furthermore, additives that promote sintering may disruptpattern formation by destabilizing the emulsions. A balance of varioussurface tensions, volatilities, viscosities is required for good patternformation, and addition of chemically active ingredients may bedeleterious to pattern formation. Finally, to ease production, additivesshould pose minimal risk of damage to production equipment or personnelowing to evolution of corrosive by products (for instance with highvapor pressure) during the process.

It has been found that the preferred sintering additives comprise a verysmall amount of acid or halide or other halogenated compound added tothe water phase prior to mixing the water phase with the organic phase.With the addition of these materials, the sheet resistance of thecoating formed from the resulting emulsion following thermal sinteringof the coating, as measured by a Loresta MCP T610 4-point probe, issubstantially less than that of coatings from emulsions prepared using awater phase that does not include the added acid, halide, or halogenatedcompound. The sheet resistance obtained when acid, halide, orhalogenated compound is added to the water phase has been found innumerous trials to be approximately 2 orders of magnitude less than thatobtained with coatings that do not include this component. Without theadded component, coating sheet resistance after thermal sintering ofabout 150° C. for 2-3 minutes yields a sheet resistance on the order of100-1000 ohm/sq. Previously, a subsequent chemical exposure step or aseries of subsequent chemical exposure steps, for example exposure to anacid or formaldehyde solution or vapor, or to acetone or other organicsolvents, was necessary to further lower the sheet resistance of suchcoatings to values on the order of 10 ohm/sq or less. In the presentinvention, sheet resistances on the order of 10 ohm/sq or less areroutinely achieved with these types of emulsions without the need forsubsequent chemical exposure steps.

The acid, halide, or halogenated compound may be added in any sequenceto the water phase with respect to other components that are present inthe water phase. Examples of suitable acids include hydrochloric acid,sulfuric acid, phosphoric acid, acetic acid, and formic acid. Examplesof suitable halides include sodium chloride, ammonium chloride, andpotassium chloride. Examples of suitable halogenated compounds includequaternary ammonium salts and ionic liquids. The concentration of acid,halide, or halogenated compound added to the water phase is preferablyin the range of 0.001M to 0.1M relative to the water phase.

In a preferred embodiment, the acid, halide, or halogenated compound isadded to the water phase prior to the addition of other components tothe water phase. In another preferred embodiment, the additive is anacid that is added to the water phase at a concentration of about 0.008Mor such that the pH of the water phase is less than 3 prior to theaddition of other components to the water phase. In another preferredembodiment, the acid is HCl.

The substrate on which the emulsions are coated may be flexible orrigid, and comprised of materials such as polymeric materials, glass,silicon or other semi-conductive material, ceramic, paper or textiles.The substrate is preferably a polymeric material such as a polyester,polyamide, polyimide, polycarbonate, polyolefin, polyacrylate,polymethyl methacrylate (PMMA), a copolymer, or mixtures thereof. Thesubstrate may have a flat surface or a curved surface, and the surfacemay be smooth or rough.

The substrate may be used directly or pretreated, for example in orderto clean the surface or otherwise alter it for improved adhesion,suitable surface tension or other characteristics. Pretreatment may beeffected by physical means or chemical means. Physical means include,but are not limited to, corona, plasma, UV-exposure, heat, infrared, orother irradiation, or flame treatment. Chemical pretreatment may beperformed, for example, with an acid, a primer, or other preliminarycoating, such as a hard-coat coating. For example, the substrate mayhave a hard-coat layer applied in order to provide mechanical resistanceto scratching and damage. A more detailed disclosure of methods ofpre-treating the substrate can be found in PCT.US2009/046243.

Pretreatment steps can be performed off-line or on-line immediatelyprior to subsequent coating, printing, and deposition steps. Suchphysical treatment of the substrate can be performed by batch processequipment or continuous coating equipment, on small laboratory scales oron larger industrial scales, including roll-to-roll processes.

The emulsion may be deposited on the substrate by coating, spraying, orother deposition methods. Coating can be performed by batch coatingequipment or continuous coating equipment, on small laboratory scale oron larger industrial scales, including roll-to-roll processes. Thecoating apparatus may be any of a variety of contact or non-contactcoaters known in the art, such as comma coaters, die coaters, gravurecoaters, reverse roll coaters, knife coaters, rod coaters, extrusioncoaters, curtain coaters, or any other coating device or meteringdevice. Coating may involve single pass or multiple pass processes.According to one embodiment of the present invention, the step ofcoating an emulsion on a surface provides a wet emulsion thickness of 1to 200 microns.

After applying the emulsion the substrate; the solvent is evaporated,with or without the application of heat. When the liquid is removed fromthe emulsion, the nanoparticles self-assemble into a network-likepattern of conductive traces defining randomly-shaped cells that aretransparent to light. A heat treatment on the order of two minutes atabout 150° C. is frequently used to increase the adhesion of the patternto the substrate.

The improved process and compositions may be suitable for, but is notlimited to, applications such as EMI shielding, electrostaticdissipation, transparent electrodes, touch screens, wireless electronicboards, photovoltaic devices, conductive textiles and fibers, displayscreens, organic light emitting diodes, electroluminescent devices, ande-paper.

Example 1

As a comparative example, an emulsion with the following formulation wasprepared without addition of an acid, halide, or halogenated compound inthe aqueous phase:

Organic Phase:

Component Supplier Weight (g) Cymel 1168 Cytec Industries 0.06 K Flex148 King Industries 0.06 Byk 410 BYK-Chemie 0.08 Disperbyk 106BYK-Chemie 0.03 Sorbitan monostearate Span 60 Sigma-Aldrich 0.08 AnilineSigma-Aldrich 0.14 Nacure 2501 King Industries 0.10 Cyclohexanone GadotChem Term. 1.70 Toluene Gadot Chem Term. 29.5 Ag nanopowder PT204 CimaNanotech 2.66Aqueous Phase:

Component Supplier Weight (g) Water — 18.7 sodium dodecyl sulfate (SDS)Sigma-Aldrich 0.005 2-aminobutanol Fluka 0.044

This formulation corresponds to a total metal loading of 5% by weight.Following coating of the emulsion on a PET substrate (SH34, SKC Corp.)at a wet coating thickness of 40 microns, drying in air, and thermaltreatment for at 150° C. for 2 minutes, the resistance of the resultingcoating was 270 ohm/sq, as determined with a Loresta GP resistance meterequipped with an ESP-type 4-point probe. When such films are thenfurther treated by immersing in a 1 M HCl solution for a duration of30-60 seconds followed by rinsing in water for about 30 seconds, sheetresistance is reduced to values in the range of 2-10 ohm/sq.

Example 2

An emulsion with the same organic phase as in Example 1 was prepared.The aqueous phase was altered relative to Example 1, such thathydrochloric acid (HCl) was added to the water at a concentration of0.008 M:

Aqueous Phase:

Ingredients Weight (g) 0.008M HCl in water 18.7 SDS 0.005 2-aminobutanol0.044

The pH of the water with added acid was 2.25. Following subsequentmixing with SDS and 2-aminobutanol, the pH of the aqueous phase was10.0. The aqueous phase was then mixed with the organic phase to form anemulsion. Following coating of the emulsion on an optical grade PETsubstrate (Skyrol SH34, SKC Corp., Korea) at a wet coating thickness of40 microns, drying in air, and thermal treatment at 150° C. for 2minutes, the resistance of the resulting coating was 3.5-4 Ω/sq, asdetermined with a Loresta GP resistance meter as in Example 1.Transparency was 70.5% as determined with a Cary 300 UV-Visspectrophotometer in the range of 370-770 nm. A primer layer was notneeded on the substrate and good adhesion to the substrate was achieved.

Example 3

In an industrial pilot run using roll-to-roll coating equipment, theformulation of Example 2 above yielded films with an average sheetresistance of 3.6 and 3.5 Ω/sq in the machine direction (MD) and thetransverse direction (TD), respectively. The process involved feedingthe emulsion formulation through a coating die at a rate of 62 ml/minonto as received SH34 PET substrate moving at a rate of 6 m/min.Following exposure to a subsequent 5 meter zone in which the organicsolvent in the coated film was allowed to evaporate, the coated film wasthen automatically fed to an on-line continuous oven with twoconsecutive heated zones of 4.5 meters each. The temperature of thefirst heated zone was controlled to 130° C. and the second zone to 140°C. As part of the same online process, the resulting film was thenautomatically wound into a roll.

Subsequent testing of the film showed that light transmission was 68%.Average cell diameter of the random cells in the coated pattern was 143microns and average line width was 15.8 microns, as determined with theaid of Image-Pro Plus 4.1 imaging software. Adhesion of the coating tothe substrate was checked by rubbing the film with an index finger, andadhesion was found to be good. The process did not require a preliminaryprimer coating on the substrate or other pre-coating treatment of thesubstrate, nor did it require post-coating chemical wash treatment stepsto yield the exceedingly low sheet resistance that was obtained.Therefore, the process for applying the coating is a highly effectiveone-pass one-coat process.

Using the same equipment and the same emulsion formulation, anothersample of as received SKC SH34 PET substrate was coated. In this run,the emulsion was fed to the coating die at a rate of 80 ml/min and theequipment line speed was 10 m/min. Following exposure to the sameevaporation zone and oven zones as mentioned above, and after on-linerolling into a completed roll, the resulting film had average sheetresistance values of 5.9 Ω/sq and 5.5 Ω/sq in MD and TD, respectively.Average cell diameter for the resulting coated pattern was 107 micronsand average line width of the cells was 13.4 microns.

Example 4

An emulsion was prepared as in Example 2, except that the acid was H₂SO₄instead of HCl and the amount of acid added was such that the pH of thesolution of water plus acid was 2.25. Final pH of the aqueous phase was9.2. Following coating, drying, and thermal treatment as in Examples 1and 2 above, the resistance of the coated film was 33 ohm/sq.

Example 5

An emulsion was prepared as in Example 2, except that the acid was H₃PO₄and the amount of acid added was such that the pH of the solution ofwater plus acid was 2.25. Final pH of the aqueous phase was 8.3.Following coating, drying, and thermal treatment as in Examples 1 and 2above, the resistance of the coated film was 23 ohm/sq.

Example 6

An emulsion was prepared as in Example 2 with HCl in the water phase,except that the amount of acid added was such that the pH of thesolution of water plus acid was 2.0. Final pH of the aqueous phase was8.9. Following coating, drying, and thermal treatment as in Examples 1and 2 above, the resistance of the coated film was 9-12 ohm/sq andtransparency was 73%.

Example 7

An emulsion was prepared as in Example 2, except that NaCl halide saltat a level of 0.008 M was added to the water phase instead of HCl. FinalpH of the aqueous phase was 10.0. Following coating, drying, and thermaltreatment as in Examples 1 and 2 above, the resistance of the coatedfilm was 25 ohm/sq and transparency was 69%. When an emulsion wasprepared with a concentration of NaCl in water of 0.01M, resistance ofthe resulting film upon drying and thermal treatment as performed inExamples 1 and 2 was 3.5-4 ohm/sq and transparency was 67%.

Example 8

An emulsion was prepared as in Example 2, except that NH₄Cl halide saltat a level of 0.008M was added to the water phase instead of HCl. FinalpH of the aqueous phase was 9.6. Following coating, drying, and thermaltreatment as in Examples 1 and 2 above, the resistance of the coatedfilm was 2.5 ohm/sq and transparency was 70%.

Example 9

An emulsion was prepared as in Example 2, except that KCl halide salt ata level of 0.008M was added to the water phase instead of HCl. Final pHof the aqueous phase was 9.5. Following coating, drying, and thermaltreatment as in Examples 1 and 2 above, the resistance of the coatedfilm was 5 ohm/sq and transparency was 67%.

The invention claimed is:
 1. A process for forming a transparent conductive coating on a substrate comprising: (1) forming an emulsion by mixing together (a) an oil phase comprising a solvent that is non-miscible with water having dispersed therein metal nanoparticles, and (b) a water phase comprising water or a water-miscible solvent and an additive for delayed sintering of the nanoparticles that reduces the standard reduction potential of the metal ion of the metal forming the nanoparticles by an amount greater than 0.1 V but less than the full reduction potential of the metal ion, wherein the additive is selected from the group consisting of a halide and a halogenated compound; (2) applying the emulsion to a substrate to form a wet coating; and (3) evaporating the liquid from the coating to cause the nanoparticles to self-assemble and form a dry coating comprising a network of electrically-conductive traces that define randomly-shaped cells that are transparent to light.
 2. The process of claim 1 wherein the dry coating has a sheet resistance less than 100 ohm/sq.
 3. The process of claim 2 wherein the dry coating has a sheet resistance less than 10 ohm/sq.
 4. The process of claim 1, wherein the additive is present in the water phase at a concentration of 0.001M to 0.1M.
 5. The process of claim 1, wherein the additive is a halogenated compound.
 6. The process of claim 5, wherein the halogenated compound comprises a quaternary ammonium salt or an ionic liquid.
 7. The process of claim 1, wherein the additive is a halide.
 8. The process of claim 7 wherein the halide comprises sodium chloride, ammonium chloride or potassium chloride.
 9. The process of claim 1 wherein the pH of the water phase is less than 3.0 after addition of the additive and greater than 8.0 when mixed with the oil phase.
 10. A process for forming a transparent conductive coating on a substrate comprising: (1) forming an emulsion by mixing together (a) an oil phase comprising a solvent that is non-miscible with water having dispersed therein metal nanoparticles, and (b) a water phase comprising water or a water-miscible solvent and an additive for delayed sintering of the nanoparticles that reduces the standard reduction potential of the metal ion of the metal forming the nanoparticles by an amount greater than 0.1 V but less than the full reduction potential of the metal ion; (2) applying the emulsion to a substrate to form a wet coating; and (3) evaporating the liquid from the coating to form a dry coating comprising a network of electrically-conductive traces that define randomly-shaped cells that are transparent to light, wherein the additive is selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid a halide, and a halogenated compound.
 11. The process of claim 10, wherein the dry coating has a sheet resistance less than 100 ohm/sq.
 12. The process of claim 11, wherein the dry coating has a sheet resistance less than 10 ohm/sq.
 13. The process of claim 10, wherein the additive is present in the water phase at a concentration of 0.001M to 0.1M.
 14. The process of claim 10, wherein the additive is a halide.
 15. The process of claim 14, wherein the halide comprises sodium chloride, ammonium chloride, or potassium chloride.
 16. The process of claim 10, wherein the pH of the water phase is less than 3.0 after addition of the additive and greater than 8.0 when mixed with the oil phase.
 17. A process for forming a transparent conductive coating on a substrate comprising: (1) forming an oil phase comprising a solvent that is non-miscible with water having dispersed therein metal nanoparticles; (2) forming a water phase comprising water or a water-miscible solvent and an additive for delayed sintering of the nanoparticles that reduces the standard reduction potential of the metal ion of the metal forming the nanoparticles by an amount greater than 0.1 V but less than the full reduction potential of the metal ion, wherein the additive is selected from the group consisting of an acid, a halide, and a halogenated compound; (3) forming an emulsion by mixing together the oil phase and the water phase; (4) applying the emulsion to a substrate to form a wet coating; and (5) evaporating the liquid from the coating to form a dry coating comprising a network of electrically-conductive traces that define randomly-shaped cells that are transparent to light.
 18. The process of claim 17, wherein the additive is a halide or a halogenated compound.
 19. The process of claim 17, wherein the additive is an acid selected from the group consisting of hydrochloric acid, sulfuric acid, and phosphoric acid. 